Inlet bell for centrifugal fans

ABSTRACT

Inlet bell for centrifugal fans having an impeller (15, 24) with a cover plate (14, 23), the inlet bell opening into the cover plate with an intervening gap (I) for recirculation air, wherein the inlet bell (11, 20) is provided with a conical inlet part (12, 21), tapering towards the impeller (15, 24), and a mouthpiece (13, 22) connected to the inlet part, and in that a circular flow guide (16, 23) is arranged at the intersection between the inlet part and the mouthpiece of the inlet bell.

The present invention refers to a novel inlet bell for centrifugal fanshaving an impeller with a cover plate with a circular inlet opening, theinlet bell opening into the cover plate with an intervening gap forrecirculated air.

PRIOR ART

Many shapes have already been designed and used for inlet bells ofcentrifugal fans. Those shapes are designed either for good performancesof the fan or for minimum cost.

For obtaining a good performance prior art inlet bells have been madevery smooth by manufacturing the pieces using spinning technique. Thismakes the inlet bells very expensive. In FIG. 1 an inlet bell of thiskind is shown, with an outlet end manufactured using spinning technique,opening into the impeller of a centrifugal fan.

Where these high costs are not accepted, simple constructions with aninlet bell in the form of a cylinder, as in FIG. 2, or as a cone, as inFIG. 3, have been used with a resulting poor performance. This is due toundesirable eddies formed in the recirculation area adjacent the inletof the impeller coverplate.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is to provide an inlet bell with a goodperformance and at the same time a low manufacturing cost.

This is accomplished with the inlet bell according to the invention,which is characterized in that the inlet bell is provided with a conicalinlet part, tapering towards the impeller, and a mouthpiece connected tothe inlet part, and in that a circular flow guide means is arranged atthe intersection between the inlet part and the mouthpiece of the inletbell.

With this arrangement according to the invention a fairly goodperformance is achieved at a very reasonable manufacturing cost for theinlet bell. The object of the flow guide means is to obtain a separationof the flow from the inlet part of the bell with as small a disturbanceof the flow as possible.

The flow guide means could be realized in several different ways.

According to a first embodiment of the invention the flow guide means isa protruding lip formed inside the mouthpiece by the tapering end of theinlet cone, which is partly inserted into the mouthpiece.

The performance may be further enhanced by providing a chamfered orrounded edge of the protruding lip, and/or forming the lip with a smallcurvature at the edge. With this arrangement the point where the flowseparates from the surface is fixed. The costs for the chamfering orrounding the edge and the forming of a small curvature on the edge of acylinder or cone is very low and easily done.

According to a preferred embodiment of the invention the conical inletpart has a cone angle α of between 25° and 50° in order to give thedesired airflow into the fan.

According to a further embodiment of the invention the mouthpiece hasthe form of a cylinder. Preferably the ratio between the diameter of thecircular flow guide means and the mouthpiece diameter d/D (see FIG. 5)lies between 0,8 and 1, and the ratio between the length of themouthpiece 1 and the mouth piece diameter D between 0,1 and 0,28.

According to another embodiment of the invention the mouthpiece has aconical outwardly tapering form, the flow guide means being a protrudinglip formed inside the mouthpiece by the tapered end of the inlet part,which is partly inserted into the mouthpiece.

According to still another embodiment of the invention the mouthpiecehas a conical outwardly tapering form, the flow guide means being theedge formed between the two conical parts secured to each other at theirrespective narrow ends, the angle (β) being at least 230°.

One advantage of the conical mouthpiece is that it provides a lowresistance to the recirculating flow entering through the gap betweenthe inlet mouthpiece and the impeller coverplate. This recirculatingflow maintains pressure stability of the fan and can increase thepressure.

As in the case of the protruding lip, the outlet edge of the mouthpiececan be chamfered or rounded and/or might have a small curvature in orderto further enhance the flow characteristics and to fix the point wherethe flow separates from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained more in detail in the followingdescription of embodiments of the invention illustrated in theaccompanying drawings, in which

FIGS. 1-3 are schematic cross sectional views of prior art inlet bells,discussed in the introductory part of the description,

FIG. 4 is a schematic cross sectional view of one embodiment of theinlet bell according to the invention, also showing part of the inletbell in an enlarged scale,

FIG. 5 is a view corresponding to FIG. 4 showing importantcharacteristic dimensions of the inlet bell,

FIG. 6 is a schematic cross sectional view of a second embodiment of theinlet bell according to the invention,

FIGS. 6a and 6b showing part of the inlet bell in two differentconfigurations in an enlarged scale,

FIGS. 7a and 7b are fragmentary views showing two different embodimentsof the flow guide means, and

FIGS. 8a, 8b and 8c are fragmentary views showing differentconfigurations of the flow guide means and the outlet end of themouthpiece.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1 a prior art type inlet bell 1 is shown with a smooth roundedoutlet part 2 opening into a coverplate 3 of the impeller 4. Thisresults in a very good performance with undisturbed flow indicated witharrows F₁, but, as discussed above, this part will be quite expensive.

In FIG. 2 the inlet bell 5 is cylindrical, which is a very inexpensivesolution, but the performance is poor, as illustrated with eddies F₂formed along the inside wall of the coverplate 6 of the impeller 7. Thesame result is obtained with a conical inlet bell 8 opening into thecoverplate 9 of the impeller 10 as illustrated with arrows F₃ in FIG. 3.

In FIG. 4, a first embodiment of the invention is shown. The inlet bell11 has a conical inlet 12 and a cylindrical mouthpiece 13, which opensinto a coverplate 14 of the impeller 15. The conical inlet 12 protrudesinto the mouthpiece 13 and the two parts are welded together or combinedin any other suitable way. The part of the inlet cone 12 protruding intothe mouthpiece forms a lip 16 acting as a flow guide means. According tothe enlarged view in FIG. 4 of the lip and the adjacent parts of theinlet cone and the mouthpiece, the edge 17 of the lip is chamfered as isalso the edge 18 of the mouthpiece. With this arrangement the pointwhere the flow separates from the surface is fixed. The flow isillustrated with arrows F₄, showing that eddies will form downstream theflow separation point. The main flow will pass over these eddiessubstantially undisturbed, resulting in a good performance. The lengthof the lip is a function of flow velocity and may be determined duringmodel tests for optimum performance. Critical dimensions will bediscussed more in detail in connection with the description of FIG. 5.

In FIG. 5, the important geometrical dimensions are inserted. The coneangle α of the conical inlet should be kept within the interval25°<α<50°. The diameter of the flow guide means, i.e. the lip 16, whichis the diameter of the smaller opening of the conical inlet "d" shouldbe less or equal to the diameter "D" of the cylinder acting as themouthpiece of the inlet bell, and preferably the ratio d/D should bekept within the interval 0,8-1. Finally, the length "1" of the cylindershould stand in a relation to the diameter "D" of the cylinder such that0,1<1/D<0,28.

In FIG. 6 another embodiment of the invention is shown. The inlet bell20 comprises a conical inlet 21 getting narrower in the direction of theflow and a conical mouthpiece 22 getting wider in said direction, whichparts are welded or otherwise connected to each other with a lip 23formed as according to the FIG. 4 embodiment, or an edge 23b, see FIG.6b. With a cone angle between 25° and 50° for each of said two conicalparts, the angle β between the walls of the two conical parts, see FIG.6a, will be at least 230° which is sufficient for the separation of theflow from the surface at the connection point between the two parts, andthe edge 23b formed will act as the flow guide means. Hence there neednot be a protruding lip. The angle β is illustrated in FIG. 6a and theembodiment without a protruding lip is illustrated in FIG. 6b.

In the embodiment according to FIG. 6 the conical mouthpiece 22 givesthe advantage of a low resistance to the recirculating flow F_(I) in therecirculation area "I" between the inlet mouthpiece 22 and the innerpart of the coverplate 19 of the impeller 24. This recirculating flowmaintains pressure stability of the fan and can increase the pressure.This embodiment also results in a good performance, as discussed above,and which is shown with arrows F₆ illustrating the flow.

As is discussed above, the configuration of the different parts of theinlet bell according to the invention can be varied for obtaining thebest possible performance at a low manufacturing cost.

As an example, in FIG. 7a wherein part of a protruding lip 25 is shown,the outer edge 26 is chamfered on one side and according to FIG. 7b theedge 27 is rounded. These two alternatives will give substantially thesame effect.

In FIG. 8a, a protruding lip 28 is shown, having a small curvature,which is realized easily and cheaply by widening the narrow part of theinlet cone. In FIG. 8b a protruding lip 29 is shown being straight whilethe outer edge 30 of the cylindrical mouthpiece 31 has a smallcurvature. This will amplify the pressure stabilising effect in therecirculation area "I" by improving the flow characteristics for therecirculation air as well as the air coming through the inlet bellaccording to the invention. In FIG. 8c an embodiment, in which theprotruding lip 28 having a small curvature and the edge of themouthpiece 30 having a small curvature, is illustrated. Theseembodiments can also be used in connection with a conical mouthpiece.

I claim:
 1. Inlet bell for a centrifugal fan, the centrifugal fan havingan impeller with a cover plate, the inlet bell having an inlet end andan outlet end, the outlet end opening into the cover plate of thecentrifugal fan, the outlet end and the cover plate defining a gap forrecirculation air, the inlet bell comprising:a conical inlet part havingan inlet end and an outlet end, the inlet part being inwardly taperedtoward the outlet end of the inlet part; and a mouthpiece having aninlet end and an outlet end, the mouthpiece being connected, at itsinlet end, to the outlet end of the inlet part, the outlet end of theinlet part being partly inserted into inlet end of the mouthpiece todefine a circular lip, the lip being disposed at an intersection betweenthe inlet part and the mouthpiece.
 2. Inlet bell according to claim 1,wherein the lip is chamfered or rounded on one side.
 3. Inlet bellaccording to claim 2, wherein the lip terminates with a curvature in adirection of flow.
 4. Inlet bell according to claim 2, wherein themouthpiece is conical, the outlet end of the mouthpiece being largerthan the inlet end of the mouthpiece.
 5. Inlet bell according to claim2, wherein an edge of the mouthpiece by the outlet end of the mouthpieceis chamfered or rounded on one side.
 6. Inlet bell according to claim 2,wherein an edge of the mouthpiece by the outlet end of the mouthpieceterminates with a curvature.
 7. Inlet bell according to claim 1, whereinthe lip terminates with a curvature in a direction of flow through theinlet bell.
 8. Inlet bell according to claim 7, wherein the mouthpieceis conical, the outlet end of the mouthpiece being larger than the inletend of the mouthpiece.
 9. Inlet bell according to claim 7, wherein anedge of the mouthpiece by the outlet end of the mouthpiece is chamferedor rounded on one side.
 10. Inlet bell according to claim 7, wherein anedge of the mouthpiece by the outlet end of the mouthpiece terminateswith a curvature.
 11. Inlet bell according to claim 1, wherein themouthpiece is conical, the outlet end of the mouthpiece being largerthan the inlet end of the mouthpiece.
 12. Inlet bell according to claim11, wherein an edge of the mouthpiece by the outlet end of themouthpiece is chamfered or rounded on one side.
 13. Inlet bell accordingto claim 11, wherein an edge of the mouthpiece by the outlet end of themouthpiece terminates with a curvature.
 14. Inlet bell according toclaim 1, wherein an edge of the mouthpiece by the outlet end of themouthpiece is chamfered or rounded on one side.
 15. Inlet bell accordingto claim 14, wherein an edge of the mouthpiece by the outlet end of themouthpiece terminates with a curvature.
 16. Inlet bell according toclaim 1, wherein an edge of the mouthpiece by the outlet end of themouthpiece terminates with a curvature.
 17. Inlet bell for a centrifugalfan, the centrifugal fan having an impeller with a cover plate, theinlet bell having an inlet end and an outlet end, the outlet end openinginto the cover plate of the centrifugal fan, the outlet end and thecover plate defining a gap for recirculation air, the inlet bellcomprising:a conical inlet part having an inlet end and an outlet end,the inlet part being inwardly tapered toward the outlet end of the inletpart; and a mouthpiece having an inlet end and an outlet end, themouthpiece being connected, at its inlet end, to the outlet end of theinlet part, the mouthpiece being conical, the outlet end of themouthpiece being larger than the inlet end of the mouthpiece, an anglebetween walls of the mouthpiece and the inlet part being at least 230°.18. Inlet bell according to claim 17, wherein an edge of the mouthpieceby the outlet end of the mouthpiece is chamfered or rounded on one side.19. Inlet bell according to claim 18, wherein an edge of the mouthpieceby the outlet end of the mouthpiece terminates with a curvature. 20.Inlet bell according to claim 17, wherein an edge of the mouthpiece bythe outlet end of the mouthpiece terminates with a curvature.